Ovarian cancer is the 5th most common cancer in women with approximately 23,000 new cases and 15,000 deaths each year in the United States. Although advances in surgery and chemotherapy have improved the survival rate, the development of resistance to chemotherapy continues to represent a challenging clinical problem in the treatment of advanced stage ovarian carcinoma. Thus, greater effort is needed to identify novel therapeutic targets and develop novel treatment strategies. Targeting signaling pathways that are dysregulated in cancer is one approach to improving patient survival. Transforming growth factor-beta (TGFβ) signaling critically regulates development and homeostasis in multiple cell types. TGFβ initiates intracellular signaling by binding to cell surface receptors (TGFβRI and TGFβRII) leading to phosphorylation of SMAD2/3, which bind to SMAD4 and translocates into the nucleus to interact with transcription factors to regulate expression of target genes. TGFβ can also signal through SMAD-independent pathways, including PI3K/AKT (Elliott, & Blobe, 2005. Role of transforming growth factor Beta in human cancer. J Clin Oncol 23, 2078-2093), which is aberrant in ovarian carcinomas. Several TGFβ signaling mediators are altered during ovarian cancer development including ecotropic viral integration site-1 (EVI1) (Nanjundan, et al. 2007. Amplification of MDS1/EVI1 and EVI1, located in the 3q26.2 amplicon, is associated with favorable patient prognosis in ovarian cancer. Cancer Res 67, 3074-3084), and TGFβRII (Sunde, et al. 2006. Expression Profiling Identifies Altered Expression of Genes That Contribute to the Inhibition of Transforming Growth Factor-{beta} Signaling in Ovarian Cancer. Cancer Res 66, 8404-8412). TGFβ mediates both tumor suppressing and tumor promoting activities by repressing transformation in normal cells and increasing aggressiveness of transformed cells through induction of epithelial-mesenchymal transition (EMT) leading to increased invasion and metastases. In ovarian cancers, proposed mechanisms for resistance to TGFβ-mediated growth inhibition include decreased expression of TGFβ receptors, repression by oncoproteins (EVI1 and SnoN), dysregulation of RUNX1 activation, and activation of additional pathways such as PKC (Elliott, & Blobe, 2005. Role of transforming growth factor Beta in human cancer. J Clin Oncol 23, 2078-2093).
Arsenic trioxide (As2O3), an effective treatment for patients with acute promyelocytic leukemia (APL) (Zhang, et al. 2001. Arsenic trioxide, a therapeutic agent for APL. Oncogene 20, 7146-7153), is active in vitro in several solid tumor cell lines, including ovarian cancer cells (Bornstein, et al. 2005. Arsenic Trioxide inhibits the growth of human ovarian carcinoma cell line. Gynecol Oncol 99, 726-729; Kong, et al. 2005. Arsenic trioxide induces apoptosis in cisplatin-sensitive and -resistant ovarian cancer cell lines. Int J Gynecol Cancer 15, 872-877; Uslu, et al. 2000. Arsenic trioxide-mediated cytotoxicity and apoptosis in prostate and ovarian carcinoma cell lines. Clin Cancer Res 6, 4957-4964). In human ovarian carcinoma cell lines, As2O3 is reported to be highly cytotoxic, inducing apoptosis, necrosis, autophagy, and inhibiting invasion (Bornstein, et al. 2005. Arsenic Trioxide inhibits the growth of human ovarian carcinoma cell line. Gynecol Oncol 99, 726-729; Zhang & Wang. 2006. Arsenic trioxide (As(2)O(3)) inhibits peritoneal invasion of ovarian carcinoma cells in vitro and in vivo. Gynecol Oncol 103, 199-206). The mechanism of action of As2O3 is presently unclear. Interestingly, in primary murine leukemia cells, As2O3 (between 2-10 μM) has been shown to degrade EVI1, a well known TGFβ signaling repressor, by the ubiquitin-proteosome pathway, while MDS1 degradation occurs via a proteosome-independent pathway (Shackelford, et al. 2006. Targeted degradation of the AML1/MDS1/EVI1 oncoprotein by arsenic trioxide. Cancer Res 66, 11360-11369). The heat shock protein-90 (HSP90) inhibitor, geldanamycin (17-allylaminogeldanamycin, 17-AAG), which is presently in phase II clinical trials (Tsutsumi & Neckers. 2007. Extracellular heat shock protein 90: a role for a molecular chaperone in cell motility and cancer metastasis. Cancer Sci 98, 1536-1539), is responsible for degrading a number of “client” proteins, including TGFβRI and TGFβRII via the ubiquitin-proteosome pathway. HSP90 directly interacts with TGFβRI and TGFβRII increasing TGFβ receptor degradation by modulating the activity of the E3 ubiquitin ligase SMURF2 (Wrighton, et al. 2008. Critical regulation of TGFbeta signaling by Hsp90. Proc Natl Acad Sci USA 105, 9244-9249). Interestingly, HSP90 is overexpressed in a number of cancers and maintains the conformational stability and function of a number of oncogenic “client” proteins which have been shown to have roles in regulating cellular proliferation, invasion, metastasis, and angiogenesis (Drysdale, et al. 2006. Targeting Hsp90 for the treatment of cancer. Current opinion in drug discovery & development 9, 483-495; Soo, et al. 2008. Heat shock proteins as novel therapeutic targets in cancer. In Vivo 22, 311-315). Dual treatment of arsenic trioxide and geldanamycin has been previously reported to have synergistic functional effects (Pelicano, et al. 2006. Targeting Hsp90 by 17-AAG in leukemia cells: mechanisms for synergistic and antagonistic drug combinations with arsenic trioxide and Ara-C. Leukemia 20, 610-619; Wetzler, et al. 2007. Synergism between arsenic trioxide and heat shock protein 90 inhibitors on signal transducer and activator of transcription protein 3 activity—pharmacodynamic drug-drug interaction modeling. Clin Cancer Res 13, 2261-2270).
TGF-β mediates differential roles depending on the stage of tumorigenesis (Elliott and Blobe, 2005. Role of transforming growth factor Beta in human cancer. J. Clin. Oncol. 23, 2078-2093). During tumor initiation, TGF-β functions as a growth inhibitor increasing apoptosis. In contrast, during tumor progression, TGF-β increases epithelial mesenchymal transition (EMT) increasing invasiveness and metastatic potential leading to a worsened outcome. Although it is clear that TGF-β function is aberrant in ovarian cancer (Hurteau et al., 1994. Transforming growth factorbeta inhibits proliferation of human ovarian cancer cells obtained from ascites. Cancer 74, 93-99) and there are rare mutations in the TGF-β receptors and SMADs in ovarian cancer (Chen et al., 2001. Transforming growth factor-beta receptor type I gene is frequently mutated in ovarian carcinomas. Cancer Res. 61, 4679-4682; Wang et al., 1999. Mutation analysis of the Smad3 gene in human ovarian cancers. Int. J. Oncol. 15, 949-953; Wang et al., 2000. Mutation analysis of the Smad6 and Smad7 gene in human ovarian cancers. Int. J. Oncol 17, 1087-1091), the mechanisms underlying the aberrations in TGF-β function in ovarian cancer remain unclear. Recently, EVI1 as well as DACH1 have been shown to be upregulated in ovarian cancers where both gene products inhibited TGF-β signaling in immortalized normal ovarian epithelial cells. Further, a DACH1 dominant negative partially restored signaling in ovarian cancer cell lines resistant to TGF-β suggesting that these aberrantly expressed genes may be partially responsible for disrupting TGF-β signaling in ovarian cancer (Nanjundan et al., 2007. Amplification of MDS1/EVI1 and EVI1 located in the 3q26.2 amplicon, is associated with favorable patient prognosis in ovarian cancer. Cancer Res. 67, 3074-3084; Sunde et al., 2006. Expression profiling identifies altered expression of genes that contribute to the inhibition of transforming growth factor-{beta} signaling in ovarian cancer. Cancer Res. 66, 8404-8412).
The role of SnoN/SkiL in tumorigenesis is complex. SnoN has been proposed to act as an oncoprotein since its expression is increased in many human tumor cell lines and overexpression results in transformation of fibroblasts (He et al., 2003. The transforming activity of Ski and SnoN is dependent on their ability to repress the activity of Smad proteins. J. Biol. Chem. 278, 30540-30547; Zhu et al., 2005. Requirement for the SnoN oncoprotein in transforming growth factor beta-induced oncogenic transformation of fibroblast cells. Mol. Cell. Biol. 25, 10731-10744). However, contrasting reports have suggested that SnoN may also act as a tumor suppressor. Patients with stage D colorectal carcinomas had decreased expression of SnoN levels particularly in microsatellite unstable cancers perhaps due to a disrupted TGF-β signaling pathway (Chia et al., 2006. SnoN expression is differently regulated in microsatellite unstable compared with microsatellite stable colorectal cancers. BMC Cancer 6, 252). Studies in lung epithelial cells indicated that SnoN acts as a positive mediator of TGF-β-induced transcription and cell cycle arrest (Sarker et al., 2005. SnoN is a cell type specific mediator of transforming growth factor-beta responses. J. Biol. Chem. 280, 13037-13046). Further, although SnoN expression is elevated in lung and breast cancer cell lines and promotes cellular proliferation, it inhibits epithelial- to -mesenchymal transition resulting in decreased metastatic potential in xenografts (Thu et al., 2007. Dual role of SnoN in mammalian tumorigenesis. Mol. Cell. Biol. 27, 324-339). Moreover, heterozygous knockout SnoN mice display increased susceptibility to chemical-induced tumorigenesis (Shinagawa et al., 2000. The sno gene, which encodes a component of the histone deacetylase complex, acts as a tumor suppressor in mice. Embo J. 19, 2280-2291; Shinagawa et al., 2001. Increased susceptibility to tumorigenesis of ski-deficient heterozygous mice. Oncogene 20, 8100-8108). Thus, depending on the cell context and the activity of other intracellular signaling pathways, the activities of SnoN/SkiL may either promote transformation or tumor suppression while inhibiting malignant progression which may contribute to the well-established dual effects of TGF-β in tumor development (Elliott and Blobe, 2005. Role of transforming growth factor Beta in human cancer. J. Clin. Oncol. 23, 2078-2093).